Embodiments of the inventive subject matter generally relate to the field of wireless communication networks, and more particularly, to techniques for phase offset estimation with multiple symbols.
Wireless communication systems can use one or more channels to transfer data between a transmitter and receivers. These communication systems can operate according to a set of standards, defined by the Institute of Electrical and Electronics Engineers (IEEE) 802.11 committee, for Wireless Local Area Network (WLAN) communication. The 802.11 standards provide a modulation and coding scheme, a packet format, and other aspects for wireless transmission. Conforming to the 802.11 standards minimizes interference between transmitting devices (e.g., cellular phones). The 802.11n standard is a proposed amendment to the 802.11 standard and is further described in FIG. 1. The 802.11n amendment addresses multiple input multiple output (MIMO) communication for increased network throughput.
FIG. 1 illustrates an example format of a packet transmitted in a MIMO environment. As shown in FIG. 1, the packet typically includes a preamble 102 and a payload 108. In some cases, multiple transmit antennas may transmit packets with the same (duplicated) preamble and a different payload. In other cases, multiple antennas may transmit packets with a different preamble and payload.
The preamble 102 comprises a signal field 103, short training fields (STF) 104, and long training fields (LTF) 106. The signal field 103 comprises information used for interpreting packet formats. For example, the signal field 103 can indicate the packet's modulation and coding scheme, bandwidth (e.g., 20 MHz, 40 MHz), number of data bytes in the payload, additional coding schemes (e.g., LDPC coding, STBC coding, etc.), number of LTFs, cyclic redundancy check (CRC) information, etc. The STF (104) can be 4 microseconds long and can be used for automatic gain control (AGC) training in a MIMO system. In some cases, the STF (104) may also be used for packet detection.
The LTFs 106 can include one to four LTF symbols. In FIG. 1, the LTF training field 106 comprises four LTF symbols 110, 111, 112, and 113. Each of the LTF symbols (e.g., 110) may be 4 microseconds long and may include training bits and a guard interval. The LTF symbols (e.g., 110) can be used for carrier frequency offset estimation and channel estimation. The number of transmitted LTF symbols can depend on the number of space-time streams (which may be related to the number of transmit chains). In some instances, the number of transmitted LTF symbols (e.g., 110) is equal to the number of space-time streams (except in the case of 3 space-time streams, where four LTFs are transmitted). In other instances, the number of transmitted LTF symbols may be greater than the number of space-time streams. For example, four LTF symbols may be transmitted for a single space-time stream. Because the LTFs 106 may be used to estimate the channel, the number of transmitted LTF symbols may not be less than the number of space-time streams. Thus, for each transmit chain, at least one LTF symbol is transmitted. As shown in FIG. 1, four LTF symbols (110, 111, 112, and 113) are transmitted for four space-time streams.
The payload 108 comprises data symbols, a service field (used for scrambler initialization), pad bits, tail bits (e.g., to indication the end of transmission), etc. It should be noted that the packet format described in FIG. 1 is exemplary. The number and length of STF, LTF, and signal fields and their order of occurrence in the packet may vary, e.g., depending on the mode of operation. 802.11n systems can operate in a variety of modes (e.g., Green Field mode) to enable backward compatibility with systems using 802.11a/b/g standards.
Data to be transmitted in an 802.11n format may be split into two or more data streams and fed into multiple transmit chains. Each of the transmit chains can modulate the data stream using orthogonal frequency division multiplexing (OFDM). In OFDM, the transmit chain modulates the data stream with one of multiple orthogonal OFDM sub-carriers (also called a frequency bin) using any suitable modulation scheme (e.g., quadrature amplitude modulation (QAM), binary phase shift keying (BPSK), etc). The orthogonality of the OFDM sub-carriers ensures simultaneous transmission of closely spaced sub-carriers without interference. The transmit chains may further process the modulated data streams and convert them into radio frequency (RF) signals. Transmit antennas then transmit the modulated data streams through a communication channel. Typically, transmit antennas also transmit a “pilot tone” for each OFDM sub-carrier. A pilot tone is a single frequency transmitted for synchronization and reference purposes.